Polymer composite materials selected and qualified for various applications, such as with primary structure applications in the manufacture of aircraft, for example and without limitation, may be evaluated for two key mechanical properties: compression-after-impact (CAI) strength and hot-wet compression strength, and more specifically open-hole-compression (OHC) strength. However, the means for increasing a composite material's CAI strength and hot-wet OHC strength have typically been counterproductive to each other. More specifically, traditional particulate interlayer toughening methods using elastomeric or thermoplastic-based polymer particles have been effective for increasing a composite's CAI strength, but not generally effective for simultaneously increasing hot-wet compression strength (e.g., hot-wet OHC) properties and, more typically, result in a tradeoff relationship with one another.
Conventional methods utilized to increase the hot-wet compression strength properties of a polymer composite may involve increasing the resin matrix crosslink density to increase the elastic modulus of the resin or by reducing the water absorption characteristics of the matrix by proper formulation of the resin's specific chemistry. Efforts associated with increasing the matrix crosslink density to increase hot-wet compression strength typically result in a composite having reduced CAI properties.
In the interest of toughening the composite matrix interlayer sufficiently to improve its CAI strength, it will be appreciated that titanium particles are irregularly-shaped (small pieces of Ti sponge) and should provide a mechanical bond within the epoxy, along with the ductility and function of a “blocker” for crack propagation. In view of the foregoing, it would be highly desirable to provide a polymer composite structure having a matrix interlayer which provides the properties of titanium but which does not significantly add to the weight of the overall structure, and also which does not negatively affect the hot-wet compression strength of the matrix interlayer.